1. Field of the Invention
This invention relates to new special polyester polyols based on selected starting materials and to their use as a binder component in two-component polyurethane lacquers (2C PUR lacquers), preferably as lacquers for large vehicles and repair lacquers for cars.
2. Description of the Prior Art
It is known that substrates of low flexibility, such as metal or wood, can be coated with two-component polyurethane lacquers based on hydroxy-functional polyesters, polyethers or polyacrylates and organic polyisocyanates. The resulting lacquer coatings are distinguished in particular by excellent hardness, very good adhesion and high weathering resistance. The chemical bases of these lacquers and coatings are described inter alia in "Lackkunstharze", Hans Wagner/Hans Friedrich Sarx, Carl Hanser Verlag, Munchen, pages 153 to 173, 1971.
However, known two-component polyurethane lacquers, mainly those based on aromatic polyester polyols, often lead to lacquer coatings having a tendency to yellow, particularly on exposure to short-wave UV light, and frequently failing to satisfy the requirements for weathering-, chalking- and yellowing-resistant coatings in car repair and large vehicle lacquering.
A typical example of coating compositions which are susceptible to yellowing are the binders according to DE-AS 1,271,867. Although lacquers based on these binders (aromatic polyester polyols/polyisocyanates based on hexamethylene diisocyanate) show no reduction in gloss, no sign of chalking nor any change in color after 2000 weatherometer hours, they undergo distinct yellowing after 1000 h exposure to short-wave UV light.
A significant improvement in the susceptibility to yellowing of 2C PUR lacquers on exposure to short-wave UV light can be obtained with the polyester polyols described in EP-A-0,318,800. However, these polyester polyols are not suitable for the production of 2C PUR lacquers for car repair and large vehicle lacquering because the drying time (predrying and full drying) is too long and the resulting lacquer films are highly elastic and, accordingly, do not have the required surface hardness.
EP-A-0,494,442 describes heat-curing one-component coating compositions which also form UV-resistant lacquers. However, the polyester polyols used in these coating compositions are unsuitable for use as the polyol component in 2C PUR lacquers for car repair and large vehicle lacquering because the degree of branching, the hardness and the OH content of the polyesters are too low to obtain quick-drying, hard lacquer films resistant to solvents and chemicals.
Accordingly, an object of the present invention is to provide new two-component polyurethane lacquers, particularly for car repair and large vehicle lacquering, which meet the following requirements:
1. Non-yellowing film surfaces on exposure to short-wave UV light
The lacquer film must withstand exposure to short-wave UV light (wavelength &lt;400 nm) for at least 1000 hours without yellowing.
2. Rapid drying at room temperature
The lacquer film should be sand-dry after about 2 h and fully dry after about 8 h (DIN 53150).
3. Very good resistance to solvents and chemicals
The lacquer film must not be damaged by typical lacquer solvents such as xylene, butyl acetate, acetone, methylethyl ketone and high-octane fuel; or by chemicals such as sodium hydroxide, sulfuric acid, wood resin, pancreatin and tar.
4. Good light and weathering resistance
The lacquer film should show no reduction in gloss, no sign of chalking nor any change in color after 2000 weatherometer hours and after outdoor weathering in an industrial climate.
5. High film hardness and mechanical stability
The lacquer film should have reached its final hardness (pendulum damping &gt;120 s), i.e., should be scratch- and abrasion-resistant, after 2 days.
6. Adequate pot life
Freshly prepared lacquers must have a pot life of at least 6 hours, i.e., their viscosity measured as the flow time from a DIN 4 mm cup should not double in 6 hours.
It has now surprisingly been found that these objects, in particular, yellowing-resistant, quick-drying, hard lacquer films with favorable resistance properties, can be obtained by using the polyester polyols according to the invention based on selected starting materials which are described in detail hereinafter as the polyhydroxyl component or as an essential part of the polyhydroxyl component in two-component polyurethane lacquers.